One commercially available ion implantation system uses an ion source that includes a source chamber spaced from an implantation chamber where one or more workpieces are treated by ions from the source. An exit opening in the source chamber allows ions to exit the source so they can be shaped, analyzed, and accelerated to form an ion beam. The ion beam is directed along an evacuated beam path to the ion implantation chamber where the ion beam strikes one or more workpieces, typically generally circular wafers. The energy of the ion beam is sufficient to cause ions which strike the wafers to penetrate those wafers in the implantation chamber. In a typical application of such a system the wafers are silicon wafers and the ions are used to "dope" the wafers to create a semiconductor material. Selective implantation with the use of masks and passivation layers allows an integrated circuit to be fabricated.
Rutherford Backscattering Spectroscopy (RBS) is a known technique for analyzing the energy and yield of backscattered ions impinging a target. Known methods of RBS are performed on stationary materials in a laboratory setting for determining the crystalline structure of the material. U.S. Pat. No. 4,967,078 to Purser entitled "Rutherford Backscattering Surface Analyzer with 180-Degree Deflecting and Focusing Permanent Magnet" discloses a RBS analyzer suitable for use in a research laboratory setting. The apparatus of the '078 patent generates a 2.0 MeV ion beam which is directed onto a sample. An RBS detector counts the ions deflected from the sample. Dedicated RBS software then generates a backscattering spectrum from the sample.
Semiconductor materials such as silicon are used in fabricating electronic devices because of their crystalline structure. Materials have a crystalline structure when their atoms are arranged in three dimensions in a regular manner, known as a crystalline lattice. Silicon wafers used in the fabrication of electronic devices are made from large single crystals of silicon specially grown for that purpose. Crystalline structure is advantageous in electronic devices because it facilitates control of the electrical properties of the device and exhibits uniform electrical properties throughout the entire material. Finally, because impurities which degrade device performance tend to collect around irregularities in the atomic structure of a material, regular crystalline structure promotes optimum device performance and yield.
An important parameter of a semiconductor wafer ion implant process is the angle of incidence of the ion beam with respect to the wafer and internal lattice structure of the semiconductor material of the wafer. The angle of incidence is important because of the role it plays in the phenomenon of channeling. Dopant depth profiles vary as a function of position on the wafer surface if the incident angle of the beam varies across the surface. U.S. Pat. No. 5,432,352 to van Bavel entitled "Ion Beam Scan Control" discloses a method for compensating for deviations in the angle of incidence between the ion beam and the semiconductor wafer during ion beam scanning. This patent concerns the use of a variable speed motor which scans the semiconductor wafer through the ion beam. The motor varies the speed of the scan as a function of the predicted angle of incidence between the ion beam and the wafer. Systems such as the one disclosed by van Bavel rely on assumptions about the precision of the wafer cut with respect to the lattice structure of the silicon crystal and assume a perfectly straight ion beam.